Joplin is based on markdown, and backslash is used as an escape character in markdown.
If you have text that contains a backslash (and you want ensure that the backslash is rendered when looking at the rendered markdown preview), there sometimes needs to be another backslash to escape it.
For example $In\frac may be rendered as: $Infrac
But $In\\frac will be rendered as $In\frac
So, it will look odd in the markdown editor view, but should look correct in the rendered view.
If seeing the extra backslashes in the editing view makes you feel icky (as it does for me as well xD) you can put the equations in inline code by surrounding them with backticks:
`$In\frac`
Alternately, a code block could be used (three backticks above and below the text)
Text that's formatted as code (inline code or code block) will be displayed exactly as is, with no extra escape characters needed.
gotcha- is there any particular action in joplin that seems to trigger this happening?
I tested entering an equation here and then doing things such as switching between the markdown and rich text editor, and making changes to the note in the rich text editor and it didn't seem to mess with the equation formatting at all
I replicated this issue and cause on the same OS and Joplin version. In the meantime, instead of using the problematic Rich Text editor, you can keep dollar signs in place how you want by using Markdown plugins for table management:
Rather than trying to use $...$ to align the equations to the left as inline text, you could align them within a $$...$$ block using LaTeX's built in flalign environment to align everything flush with the left.