I still often use MPEG Streamclip in Mojave to losslessly trim unwanted material off from DJI drone and other raw videos before archiving them. I don't know what to do after updating to Catalina+ -- MPEG Streamclip seems to work OK from VMware running OS X 10.6-11-14 but maybe I'll switch to QT or some other native app by then. But I digress.
It seems the bitrate info is just calculated slightly differently. I did a quick test with a 4K 4min 24sec 12 frame .mp4 by trimming it with Mojave's QuickTime Player 10.5 (QT), QuickTime Player Pro 7 (QT7) and MPEG Streamclip:
Original 59.95 Mb/s reported by QT and MPEG Streamclip - QT7 reports 60.0 Mb/s
Trimmed to 30 sec from 20-50 sec by QT, QT7 and MPEG Streamclip:
QT 60.13 Mb/s reported by QT and MPEG Streamclip - QT7 reports 61.8 Mb/s
QT7 60.08 Mb/s reported by QT and MPEG Streamclip - QT7 reports 62.7 Mb/s
MPEG Streamclip 60.0 Mb/s reported by QT and MPEG Streamclip - QT7 reports 59.99 Mb/s
All apps trimmed that in 1-2 sec so I presume it was lossless (maybe the very start and end beyond GOPs were re-encoded (*) with QT and QT7), and at a quick glance the quality seemed intact.
QT7 reports slightly different bitrate compared to QT and MPEG Streamclip. Trimmed clips show slightly different bitrates in all apps.
(*) MPEG Streamclip always losslessly trims to the GOP. Some of my pet peeves with QT Player is that a) it shows fractions of a second instead more clearly showing frame numbers (and GOPs). Sure, I can calculate the frame number from the fraction by multiplying it by the frame rate but I'd prefer to see the actual exact frame numbers. b) Usually I'd like to edit to the GOP so there is no re-encoding induced quality loss anywhere.