Thank @RonShepard for the suggestion.
Indeed, as long as I can confirm that the behaviour of gfortran -Ofast regarding .eqv. illustrated in my example is unexpected or questionable, then I will stop worrying and will not strive to circumvent it.
This is quite different from the problem discussed in another thread, where the behaviour of gfortran -Ofast regarding floating point computation is totally expected and I understand why it happens, although I would like workarounds to “fix” it.