HTMLHelp (API) issue (Win or Office 365)

[PaulZak]

Today I found that my Excel file's existing code, which I use to open our .chm help file, no longer works.
There are no errors, but the help file does not open.
I can manually open the .chm file from Win Explorer.

We are running Excel 365 on Win10.
The code has worked as recently as Feb 22, 2021.
Today I asked a few other people to run the code on their PCs and they experienced the same problem (no error but CHM file doesn't open)

Testing:
On a backup PC, I ran restore point, rolled back to Feb 22, and then the file ran fine in Office 365.
So this looks like it's an issue with a recent Microsoft update for Office (or Win10?).

CODE SUCCEEDS:
Excel - Version 2101 (build 13628.20488)
Win10 - 10.01.19041 (Build 19041)

CODE DOES *NOT* SUCCEED
Excel - Version 2102 (build 13801.20266)
Win10 - 10.01.19042 (Build 19042)

In addition to reverting Excel back, my restore point removed these recent WIN updates.
But they are installed on another PC running Excel 2010, and code works, so they are likely not related.
KB4023057
KB4601319
KB4601050

Has anyone seen this problem and if so, have any ideas to correct it?

Has this happened before and MS eventually releases a fix/update?