Your old antenna may be very directional, and the weak signals may be fading in and out. One way I have checked for usable signals is with a common rabbit ear antenna that comes with most sets. The antenna length should be rather short, approximately 11 inches for DT-25. To be exact, divide 5904 by the frequency. WYFF wold be 5904/741.25 = 7.96 inches, each side. The dipole is not very directional. Open it flat with it set for about 9 or 10" perhaps as short as it will go, and aim it about mid way between the stations. (Did you move the old antenna during your scans?)
The other potential is a multipath problem that causes the pixelizatiion. The rabbit ear antenna potentially will worsen any multipath and that will become more obvious.
You don't say what brand receiver you have. Some drop any channel not received during each scan, some retain previously locked channels. The key is to find a place where all channels can be received, then don't scan any more! Observe what you receive for a while (couple of days) and see if the pixelization continues. You don't mention the time frame for your test, but don't forget the stations still are off the air rather frequently. During the past week 25 was off for a power change. Just prior to that 56 was off for several days. WYFF-DT is subject to power outages.
If you find the temporary dipole does a better job of getting all available stations, then I would recommend a UHF only antenna, such as the Channel Master 4-bay bow tie. Good gain, and a reasonable reception pattern. (The temp dipole is not intended to be permanent.)
I don't believe the receiver is the cause of the problem of dropping stations...but hopefully those with more technical experience will offer advice. I would be suspicious of the older antenna, leadin, and any amplification and splitters. Don't judge by VHF reception of analog signals; UHF analog patterns will be more indicative of potential for UHF DT signals.
Let us know what you are finding as you continue the search.