When the user attempts to change the command assigned to a VoiceOver gesture, although it appears to have been selected correctly, the previous command is not always removed, meaning VoiceOver has two commands assigned to a single gesture. This causes issues when the user performs the gesture.
Enable VoiceOver.
Go to Settings > Accessibility > VoiceOver > Commands > Touch gestures.
For example, take the one finger triple tap gesture, by default set to 'Perform Long Press'.
Select it, and choose 'Intelligent Image Description' to be assigned to the gesture instead.
Select to replace the existing command in the dialog box that pops up.
Browse the list of commands screen and find the one finger triple tap.
It appears to confirm that your new command has indeed been assigned.
However select it and browse the list of commands again.
You will see that both the original 'Perform Long Press' and the newly selected 'Intelligent Image Description' commands are selected.
If left this way, the experience is erratic, with VoiceOver sometimes performing one of these commands, and sometimes the other one.
Note: It is irrelevant if the outgoing command is the default command or one you assigned yourself previously.
First deselect the old gesture, then go back in and select the new one.