Negative Bias from Voltage Regulator

As discussed in the comments, you have about a 0.5 Ω resistance in your ground wiring. You are losing about 0.6 volts when under load. Ohm's Law (V=I*R ==> I=V/R) says I=~0.6/~0.5 = about 1.2 amps flowing through that ground. It all adds up.

Fix the resistance in the ground wiring and you'll fix your voltage drop.

I've seen a cute trick performed inside a 1KV PMT supply regulator. Rather than using an expensive 1500V transistor plus a fancy high-side driver circuit, they used a bunch of transistor-output opto isolators in a series chain. They were wired as a HV shunt-regulator.

Choose devices with sufficient collector voltage (divided by N.) Fairchild H11D3 can take 300VDC, with 5KV isolation. Apply your LED current (also a series chain,) and you set the transistor operating point (current.)

The LED input is isolated, so the trick should also work for high-side pass-regulators as you show above.

Note that 2.5mA and 450VDC is over one watt. Your 200K resistors must not be tiny or close-spaced. And, if using an opto-iso chain, employ enough series devices to spread out the half-watt, keeping each DIP package milliwatts low enough.

I find this:

Jim Williams, "Feeding and Reading the APD:" PDF (Lin Tech Inc.)