Seam sealer bubble

[WhiteLighting21]

I wash washing my truck today and noticed this on the seam sealer on the front passenger door, what could cause this? I can push it in, very soft. I owned this truck since new, only 14k, garaged kept and hand wash weekly.

 

[HSKR R/T]

I wash washing my truck today and noticed this on the seam sealer on the front passenger door, what could cause this? I can push it in, very soft. I owned this truck since new, only 14k, garaged kept and hand wash weekly.

Has probably been there the whole time