front derailleur slipping?
This sounds like your cable has stretched. For the front derailleur, you have to apply tension to the cable to get it to move to the right ("harder gear") if the cable lengthens (or slips), by even a millimeter or two, it can cause the front tuning of the gear system to get out of whack with the result being the chain dropping to the easier gear(s).

Shift all the way to the left, loosen the bolt (or nut) holding the front derailleur cable, put tension on the cable and retighten. I suspect this will fix things.

Chainring wear could be at play here, but they would really have to be worn down. The chain angle you see is probably the natural movement when the chain is supposed to be shifting.

Also check alignment: derailleur should be be about 2mm above large chainring, curve roughly matching, and aligned in the same plane as the chainring.

Happy riding.