Steps to reproduce:
1. press ALT - mnemonics are shown
2. release ALT - mnemonics disappear
3. press ALT - there are no mnemonics shown
4. release ALT
5. press ALT - mnemonics are shown
6. release ALT - mnemonics disappear
Even though mnemonics are not shown in step 3 they still work. To test it press F to open File menu when ALT is down in step 3.
It looks like this issue is related to Windows platform only.
1. press ALT - mnemonics are shown
2. release ALT - mnemonics disappear
3. press ALT - there are no mnemonics shown
4. release ALT
5. press ALT - mnemonics are shown
6. release ALT - mnemonics disappear
Even though mnemonics are not shown in step 3 they still work. To test it press F to open File menu when ALT is down in step 3.
It looks like this issue is related to Windows platform only.