The LC100A is a fairly popular and cheap LC meter available on aliexpress and similiar sites. It is advertised as an up to 1% accurate device (depends on range) with a resolution of up to 0.01pF and 0.001uH. Unfortunatly he device I received had errors up to 50% and was thus not usable at all.
Thanks to a comment on aliexpress I found out that the issue was low quality components for the LC resonance circuit, likely the manufacture substituted them because they were cheaper. I think the main issue was that the max frequency was well above the rating of the inductor, so it's effective inductance dropped and threw off the measurements.
I replaced the parts with the best I could find (part names below) and the performance is within spec now. During my research I found a manual and a low-res schematic; I've up-scaled the latter manually and have both files attached below. The pictures are watermarked to prevent the seller from using my pictures.