The Laserfiche On-Prem system, when using workflow's 'Assign Field Values', in the Advanced Properties there is an option to override default values that looks like this:

In my experience when using this, the field can be removed from an entry if there is no value in it.  This is useful as it will only keep the extra fields if there is a value in it.

In the LFcloud this advanced option does not seem to exist.  So, via workflow, when you take an entry that has TemplateA assigned then assign TemplateB (this template contains some different fields compared to TemplateA), TemplateA's fields are still left in the unassigned to a template list of fields at the bottom (see screenshots).

Example - TemplateA

Then assign TemplateB: