I've got a reply from support,
Thank you for contacting Maplesoft, and for your email.
We appreciate your feedback on this, and have heard similar reports from customers in the past.
This limitation of Maple is known and is documented in our help pages. Relating to temperature the following is stated in the page
Expressions with a unit are represented in Maple as a product of the expression with a function call, which can be obtained by calling the procedure "Unit". This means that Maple cannot represent the expression "0" with a unit: it would be represented as, for example, "0*degC", or internally, "0 . (Unit(degC))", but "0" multiplied by any function call immediately simplifies to "0". This particularly comes up with temperatures sometimes, when dealing with empirical temperatures rather than with temperature differences. This can be a reason to use Temperature objects instead of regular unit expressions.
I am sorry for any confusion this has caused. Let us know if you have any further questions or concerns.
Manager Customer Success