Polyjuice changes you into a COMPLETE copy of the person, right down to your hormones.
Grindelwald has been an alpha his whole life and thus knows exactly zero of the tricks omegas use to mask their scent and weather their heats
therefore scheduling a meeting with perpetually terrified young alpha Credence Barebone mere hours before his heat is due to hit (not that Gindelwald knows this, being unaware of what the signs are), with the first hit of the alpha's scent morphing 'mere hours' into 'right the hell now' is....an error.
basically, Grindelwald suddenly experiencing an omega's heat for the first time, with all the emotions and panic and sheer terror that probably comes with when it's never happened to you before, and Credence (who himself has never had anyone, much less an omega) is the one fate deigns to place him with. Whether this ends well for either party involved is up to you~
Credence/Grindelgraves, ABO-verse, the real Graves is an omega OOPS
Grindelwald has been an alpha his whole life and thus knows exactly zero of the tricks omegas use to mask their scent and weather their heats
therefore scheduling a meeting with perpetually terrified young alpha Credence Barebone mere hours before his heat is due to hit (not that Gindelwald knows this, being unaware of what the signs are), with the first hit of the alpha's scent morphing 'mere hours' into 'right the hell now' is....an error.
basically, Grindelwald suddenly experiencing an omega's heat for the first time, with all the emotions and panic and sheer terror that probably comes with when it's never happened to you before, and Credence (who himself has never had anyone, much less an omega) is the one fate deigns to place him with. Whether this ends well for either party involved is up to you~