--text follows this line--
The problem can be reproduced with the `test.tex' file attached below. When
I tried to add more characters to the `align' environment in `test.tex',
the syntax highlighting becomes extremely unstable, switching between math
and plain text highlighting.

Best,
Ruiyang

Emacs  : GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Package: 12.2.0
current state:
==============
(setq
 AUCTeX-date "2019-10-31"
 window-system 'w32
 LaTeX-version "2e"
 TeX-style-path '("~/.emacs.d/auctex"
"c:/Users/ywwry/.emacs.d/elpa/auctex-12.2.0/style"
"c:/Users/ywwry/.emacs.d/auctex/auto"
"c:/Users/ywwry/.emacs.d/auctex/style" "auto" "style")
 TeX-auto-save t
 TeX-parse-self t
 TeX-master t
 TeX-command-list '(("TeX" "%(PDF)%(tex) %(file-line-error) %`%(extraopts)
%S%(PDFout)%(mode)%' %t" TeX-run-TeX nil (plain-tex-mode ams-tex-mode
texinfo-mode) :help "Run plain TeX")
      ("LaTeX" "%`%l%(mode)%' %T" TeX-run-TeX nil (latex-mode doctex-mode)
:help "Run LaTeX")
      ("Makeinfo" "makeinfo %(extraopts) %t" TeX-run-compile nil
(texinfo-mode) :help "Run Makeinfo with Info output")
      ("Makeinfo HTML" "makeinfo %(extraopts) --html %t" TeX-run-compile
nil (texinfo-mode) :help "Run Makeinfo with HTML output")
      ("AmSTeX" "amstex %(PDFout) %`%(extraopts) %S%(mode)%' %t"
TeX-run-TeX nil (ams-tex-mode) :help "Run AMSTeX")
      ("ConTeXt" "%(cntxcom) --once --texutil %(extraopts) %(execopts)%t"
TeX-run-TeX nil (context-mode) :help "Run ConTeXt once")
      ("ConTeXt Full" "%(cntxcom) %(extraopts) %(execopts)%t" TeX-run-TeX
nil (context-mode) :help "Run ConTeXt until completion")
      ("BibTeX" "bibtex %s" TeX-run-BibTeX nil (plain-tex-mode latex-mode
doctex-mode ams-tex-mode texinfo-mode context-mode) :help "Run BibTeX")
      ("Biber" "biber %s" TeX-run-Biber nil (plain-tex-mode latex-mode
doctex-mode ams-tex-mode texinfo-mode) :help "Run Biber")
      ("View" "%V" TeX-run-discard-or-function t t :help "Run Viewer")
("Print" "%p" TeX-run-command t t :help "Print the file")
      ("Queue" "%q" TeX-run-background nil t :help "View the printer queue"
:visible TeX-queue-command)
      ("File" "%(o?)dvips %d -o %f " TeX-run-dvips t (plain-tex-mode
latex-mode doctex-mode ams-tex-mode texinfo-mode) :help "Generate
PostScript file")
      ("Dvips" "%(o?)dvips %d -o %f " TeX-run-dvips nil (plain-tex-mode
latex-mode doctex-mode ams-tex-mode texinfo-mode) :help "Convert DVI file
to PostScript")
      ("Dvipdfmx" "dvipdfmx %d" TeX-run-dvipdfmx nil (plain-tex-mode
latex-mode doctex-mode ams-tex-mode texinfo-mode) :help "Convert DVI file
to PDF with dvipdfmx")
      ("Ps2pdf" "ps2pdf %f" TeX-run-ps2pdf nil (plain-tex-mode latex-mode
doctex-mode ams-tex-mode texinfo-mode) :help "Convert PostScript file to
PDF")
      ("Glossaries" "makeglossaries %s" TeX-run-command nil (plain-tex-mode
latex-mode doctex-mode ams-tex-mode texinfo-mode) :help
       "Run makeglossaries to create glossary\n     file")
      ("Index" "makeindex %s" TeX-run-index nil (plain-tex-mode latex-mode
doctex-mode ams-tex-mode texinfo-mode) :help "Run makeindex to create index
file")
      ("upMendex" "upmendex %s" TeX-run-index t (plain-tex-mode latex-mode
doctex-mode ams-tex-mode texinfo-mode) :help "Run upmendex to create index
file")
      ("Xindy" "texindy %s" TeX-run-command nil (plain-tex-mode latex-mode
doctex-mode ams-tex-mode texinfo-mode) :help "Run xindy to create index
file")
      ("Check" "lacheck %s" TeX-run-compile nil (latex-mode) :help "Check
LaTeX file for correctness")
      ("ChkTeX" "chktex -v6 %s" TeX-run-compile nil (latex-mode) :help
"Check LaTeX file for common mistakes")
      ("Spell" "(TeX-ispell-document \"\")" TeX-run-function nil t :help
"Spell-check the document")
      ("Clean" "TeX-clean" TeX-run-function nil t :help "Delete generated
intermediate files")
      ("Clean All" "(TeX-clean t)" TeX-run-function nil t :help "Delete
generated intermediate and output files") ("Other" "" TeX-run-command t t
:help "Run an arbitrary command"))
 )


-- 
Ruiyang Wu

Graduate Student
Department of Mathematics
University of Arizona
Tucson, AZ, 85721    USA