Thanks for sharing the details, Lucas! Please try to switch to a different IME temporarily. Here's how:
Go to System Settings then Keyboard then Input Sources
Add Zhuyin – Traditional Chinese (Taiwan) under Microsoft Input Method if available, or Pinyin – Traditional
Switch input sources with Control + Space and test again in Excel. The Microsoft IME variant currently avoids the spacebar issue.
See if this helps. If you need further assistance just let me know.
Best regards,
Kimberly