I have a Dell Latitude 7390 laptop that I am trying to Daisy Chain three Dell P2219HC monitors to it via USB-C and DisplayPort.
Out from Latitude 7390 via USB-C into P2219HC 1 via USB-C > out of P2219HC 1 via DisplayPort into P2219HC 2 via DisplayPort > out of P2219HC 2 via Displayport into P2219HC 3.
This seems to work, but the third P2219HC is showing up with resolution 1600x900 and then rest of 1920x1080. My understanding is this should work with 1920x1080 up to four monitors.
I have tried updating video card drivers, bios, etc. but still same issue. Any suggestions?
Connecting to the first P2219HC via USB-C is cutting your total available bandwidth down to half of a full DisplayPort 1.2 channel, which is why you only have enough for dual 1080p displays with some bandwidth left over to run the third at a lower resolution. USB-C docking stations such as the WD15 only support dual displays up to 1080p for this very reason. I wrote a long post about the various operating modes of USB-C, including its impact on display options, in this thread if you're curious. If you switched to a USB-C to DisplayPort cable to connect to the first display, you'd be dedicating that entire first link to video and therefore doubling the video bandwidth leaving your system, at which point your setup should work -- although that means that if you wanted to use the USB ports on that first display, you'd need to connect a separate USB cable from the display to your system to carry that data. Also, this may or may not affect your use case, but a while ago I tried to daisy-chain U2415 displays using regular DisplayPort all the way, and I never got a three-display chain working properly even though there was no technical reason it shouldn't have. I ended up using a two-display chain and then direct connecting the third.
And as for four displays, although the cable switch I suggested above would give you enough bandwidth to run four 1080p displays, the Intel GPU in that system only supports up to three independent displays active at any given time, counting the built-in display. So even running three external displays would require you to disable the internal display, and running four would not be possible without resorting to something like DisplayLink indirect display technology, but that has several drawbacks.
In addition to my answer above, the manual for the P2219HC available on support.dell.com (direct link here) says right there on Page 14 that you can only use 1 external display via MST (daisy-chain) if you use USB-C, but up to 3 if you use DisplayPort. ("External display" in that context means external to the first display.)