Fonts changing in files when opened in Illustrator 29.5.1
Existing files created in illustrator (on a different desktop) when opened in v29.5 & v29.5.1 display incorrect fonts - the font is changed to a completely different font (e.g. Myriad Pro will be changed to Times New Roman) throughout the document. All the text in that font family is changed, but other fonts used in the document will display correctly. The font is installed correctly on the system and the changed font can be changed to the correct font by selecting the font from the illustrator font list. The thumbnails display correctly in Bridge before opening in illustrator. Reverting illustrator back to v29.4 resolves the problem

The team reports the Beta, starting with 29.7.0.88, should have a fix for this.
This particular build introduces a lot of WIP snapping-related changes, so if your work depends on this, be mindful of it. However, before this fix gets pushed into general build, having the confirmation it actually works would be great — so please check it, if possible, and comment back.
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Howard davies commented
2 of the source files are attached below along with the screenshots showing how they display on the different versions of Illustrator. These files were created on a different MacBook and opened on my macBook as shown - note this is the same file opened on the same desktop using different version of illustrator. If the fonts are corrected and saved on v29.5.1 then the display correctly after that
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Howard, please share the files that get altered with the team for reviewing. You can do it here in the comments if you are OK with sharing them publicly, or send them to sharewithai@adobe.com (in this case please mention the link to this report for tracking purposes: http://illustrator.uservoice.com/forums/601447/suggestions/49871712)
It’d be helpful if you shared some screenshots of how it looks originally and how 29.5.X displays it, with the Character panel visible.