could be possible that GWR and/or has a good relationship with Angel Trains (owner of Class 15x/16x fleets) and thereby has managed to negotiate a favourable deal. But that’s as much as I will say at this time.
There is a lot of commercial logic in this, especially if the leaseco has terms where X quantity of trains should be available, and there are now problems sourcing parts. You get into the realm of remedy and compensation (in legal and contractual terms, rather than operational), and the contractual remedy might be an offer involving something else.
I guess when off lease the 175s produce no income for leaseCo (Angel), and if previous user has to fix them to a standard for return, or compensate for returning in below contractual return condition, then they are available to be used, or the repairs are funded to allow them to be used.
From a commercial point of view, what LeasoCo doesn't want is glut of available units, an a price race to the lowest to find a user, but if nothing else is available (and there aren't lines of store DMUs, just a few odd ones) then getting a deal before more get retired and come back onto the market makes sense.