When a user's primary language is set to Japanese or Chinese and they use the corresponding keyboard on iOS, inputs with a fixed decimal place don't work correctly. This is a limitation of the third-party input masking library Bubble uses for decimal inputs — a fix causes even worse "jumpy" input behaviour, so this keyboard combination is intentionally not supported for decimal inputs.
What you might notice
A decimal input field doesn't accept values correctly when using a Japanese or Chinese keyboard on iOS
Input behaviour is unpredictable when the device's primary language is set to Japanese or Chinese
Workaround
There's no current workaround for this specific combination. As an alternative, consider using a standard text input and validating or converting the value in your workflows.
This limitation originates from the inputmask library used for decimal inputs. You can track progress on the underlying issue here.