[Charts] Unable to merge manually configured units

In Charts, it’s possible to freely configure the units for each time series.
However, I’m having trouble when trying to merge a manually set unit with the default unit.
For example, in the screenshot, the upper time series is a calculated value added using “Create calculation.” I manually set its unit to “°C,” and attempted to merge it with the lower time series using “Merge unit.”
But Charts recognizes them as different units, so the merge doesn’t work properly.
 

To work around this, I have to manually reassign the unit “°C” to the lower time series—even though it already has the correct unit. This is quite time-consuming.
If I'm doing something wrong, I would really appreciate it if you could point it out.
If this behavior is actually by design, I hope it can be improved in the future.