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Seems like a combination of these two known bugs:

286 : Type: Game Logic | Status: FIXED | Severity: MINOR
You can claim Sole Defender of the Faith title more than once without losing it.
Reported In: 1.04 | Correction planned In : 1.05 | Corrected In : 1.05

321 : Type: GAME LOGIC | Status: CONFIRMED | Severity: NORMAL
SDOTF does not work for CRC or Reformed countries
Reported In: 1,05 | Correction planned In :- | Corrected In :-
 
In fact, for reformed (which is the one I have tested) it is even worse, it works in one way but not the other. I.e. the DoTF button is highlighted when you enter the religion screen and you can press it and the game does deduct 1000d from your treasury when you press it; but your army moral does not increase. Whether your tech cost increase (as it should) I do not remember.

An easy way to fix it would be to have the button greyed when your religion is such that a DoTF title has no impact on your nation's characteristics. Another solution would of course be to implement those characteristics when you are reformed.