Losing the keyframe at the end of a cut in copy mode

[eumagga0x2a]

We've got a very good minimal testcase, uploaded to https://we.tl/LCouZIDAFu for the first keyframe of the segment after a cut getting lost:

STR:

• Load ted_20170530.mkv in Avidemux
• Set marker A to I-frame at 00:00:09.011
• Set marker B to I-frame at 00:00:11.096
• Delete the chunk in between
• Save the video in copy mode
• Play the saved video

Actual Results:
The I-frame at the end of the cut is missing, resulting in severe video artifacts.

Reference: Weiterer Fehler in Avidemux.