Different to OP, I can confirm this bug happens on v.0.42.5 on MacOS with Traditional Chinese (Chanjie) and under the following circumstances: After a character has been selected, but suggested character is still awaiting for selection or cancellation. Anytype doesn’t think the input has been completed.
My explanation of the cause to this bug for both my scenario and OP’s.
This bug happens with IME selection. Anytype couldn’t understand if the input of characters has been finished, thus doesn’t prompt creation.
Current resolution
Input a space (spacebar) after the name of tag and it should return to normal. The prompt will display. Then the extra space can be deleted if preferred.
Additional information
Similar behaviours with IME input method had been reported for search pane and library. (Although I can’t find them now)