Hey! This isn't something I ran into when using pwntools, more like something I noticed from the source code (hence the lack of debug output etc.)
As of writing this, the most recent version of pwntools define the offsets for amd64's SigreturnFrame as follows:

Notably, sigmask is at offset 240, and __reserved is defined as being 8 bytes big. However when you look at the linux kernel source code:

https://elixir.bootlin.com/linux/v6.10/source/arch/x86/include/uapi/asm/sigcontext.h#L376

The __reserved field is actually an array of 8 u64s, which is a total of 64 bytes, causing the sigmask field to be 56 bytes short of where it should be. You can also verify this by debugging the amd64 kernel:

While this isn't a massive issue, as most people wouldn't care about sigmask, I still felt like I might as well bring it up, and you can decide whether its worth fixing.