Is it plagiarism to reuse your own full-text material from a previous conference in a new conference?
Yeah, copying 25% of a prior paper without attribution is definitely self-plagiarism.  Report it to the program chairs of the conference, and let them sort out how to manage the problem from there.

Additional information, for any coming from different publication cultures: electrical engineering / computer science conferences are serious publications that take originality very seriously, and IEEE policy on the matter is quite explicit.