I’ve found the problem – it appears that PLAY v6 is not compatible with Dorico when using Symphonic Choirs. Because of a completely separate issue with this version, I “downgraded” back to v5 some months ago (shortly before switching to Dorico from Sibelius, I believe) and this is the reason why SC works fine on my system. There’s a new thread on this now Computer für Audioanwendungen - WaveLab - Steinberg Forums
If the issue on the Dorico side is indeed to do with output of tempo information then it only affects PLAY 6.