Are 0 values in numeric subfields considered NULL?

The issue you linked is completely unrelated as it was a feeds plugin issue with the other one. You're getting the correct value out of the db (0.00) so I would assume this issue is with the tamper module. Without knowing what the error is, I can't really help point you to the problem. You can probably debug what data is somewhere in here: https://git.drupalcode.org/project/tamper/-/blob/8.x-1.x/src/Plugin/Tamp... but ultimately you should identify what error is being thrown.

Thanks for the quick response.

On a closer look, I think the empty token problem is not related to the Tamper module. Rather the Tamper error is probably thrown due to a missing / empty value that is expected to be delivered by the custom_field subfield token.

I have created a short video showing the problem, which may illustrate what I am talking about. I show here that the subfied token value is empty if the subfield value is set to 0.

Odd cause I'm not seeing this issue when you use the same tokens in the manage display type 'Custom template'. If you have token module installed and browse the custom_field tokens for the decimal value, you should see it render 0.00 as it should. In ECA though maybe its doing something different with the tokens? Could you try appending ':value' to the token(s) you're using?

E.g. [numeric_field:number_1:value]

If the above doesn't work, maybe you can split the Get custom field value into 2 separate parts and target the property directly vs. just the field.
Example: field_custom_numeric_field.number_1 as token number_1 AND field_custom_numeric_field.number_2 as token number_2. It seems like #5 though should be working though. You might ask jurgenhaas in the ECA slack channel otherwise if he has any clues.

Closing this as it was resolved via alternate token settings in ECA.