I'm making some custom low signal 1:50 step up transformer, and I noticed something interesting. My LC meter displays correct primary inductance when no secondary is present on a core. With the partially wound opened secondary its reading is few orders lower. My only guess is that it uses a relatively high frequency so the secondary becomes shorted by its own capacitance.
As some pieces of information are not known, I guess only this.
You are measuring the inductor at a frequency (10 kHz or more, probably at a "high" resonant frequency) as pointed out in the picture (abs(impedance) measured).
In red, a pure inductor. In blue, with a coupled secondary.
To make sure, perhaps measuring input impedance with a scope would be more useful.
It’s not the best to use a “random multimeter” when doing transformer prototyping. Get a proper LC meter. Doesn’t have to be expensive, just decent enough to do L/C/R/ESR measurements at various frequencies like 100, 120, 1k, 10k Hz. The DER EE DE-5000 would be a suitable meter I think, if you don’t have a big budget. I’ve used it to measure various transformers and the readings looked reasonable.