Hello! I cannot fully explain what is happened with the sds-page and the blot, but I have some suggestions.

First of all, which expression system do you use for the Spike? This is important, because the level of glycosylation (and molecular weight, respectively) can differ in different hosts. For example, the S1 subunit could be 120 kD in eukaryotic cellls and 80 kD in baculovirus insects cells (https://www.nature.com/articles/s41598-021-89242-z).

The disappearance of the 120 kD band in the immunoblot may be explained by the fact that your mAb doesn't interact with that band. It also seems unlikely that the 80 kD band is the S2 subunit, since RBD is located in S1, and your anti-RBD mAb should not be able to recognize S2.

In general, it would help if you could attach some images of your sds-page and blot

Are these from two separate SDS-PAGE gels (each prepared using the same denatured samples) or from the same one (e.g using Biorad Stain Free gels to visualise the gel before transferring)? Was the protein reduced as well as denatured before loading on the gel? How was this protein that you are analysing expressed and purified?

In my opinion, looking at other literature on this it is most likely that the 180 kDa protein represents total spike protein, while the 120 kDa band is likely the glycosylated form of the S1 subunit protein and the 80 kDa protein the form of the S1 protein lacking glycosylation. You can easily check if this is the case by using PNGase F to remove the glycans from your protein and then comparing it on the same gel to a "mock" treated sample of the protein (i.e adding water instead of PNGase F). You should see the 120 kDa band disappearing in the PNGase F sample and appearing around 80 kDa.

As mentioned previously, it would be very helpful if you could attach some images!