The condition will be inactivated at runtime, not in the Page Composer. What the new error message in version 12 is telling you is that previously you had an unresolvable Model condition dependency — a Model that’s processed server-side had a “Field from another Model” condition on a Model processed client-side, which meant that the source Model could not process that condition, so Skuid inactivated the condition and moved on.

To resolve this error, you should go find Model XXX in the Page Composer and look at its Conditions. One of those Conditions depends on a Model YYY which is processed client-side. You may need to switch 1+ models involved to both be processed client-side or both be processed server-side in order to resolve this.