The long on followed by the short off if an indication that the total bulb current on the right side is below normal. I assume that the left side has the normal on-off pattern?
Given that the tail light bulb appears to be illuminating I think you might have a failed ground path issue and that the return path for the signal light bulb may actually be occurring through the tail light bulb in series. The frame of the tail light assembly has to be solidly grounded to the body to form the return path for the signal light bulb. The wiring diagram infers that there is jumper wire on the signal light housing to form the ground path.
10 years ago I did a bare body restoration on my 140 with a new coat of paint over everything. As I re assembled the car I was testing out the individual electric systems. When it came to the running lights, signal lights and brake lights I had much electrical weirdness. All of this was traced to a fresh coat of paint interfering with the ground path. Scratching off the paint for a good ground resolved everything.
If you had corrosion on your tail light housings I expect that you have other electrical issues at the grounds and those terminal blocks. Vintage Volvos are not spectacularly water resistant. Careful inspection and some preventative maintenance may be in order. Refer to Mr. Kwas' discussion of zinc bearing grease
https://www.sw-em.com/anti_corrosive_paste.htm