If you turn the game off does the flipper fall back down on it's own? If it doesn't you can figure it is a mechanical problem. If something was really blown. I would figure that flipper would always be stuck on. I'm betting you have a mechanical problem. Maybe a mushroomed plunger or not enough vertical play in the flipper shaft. If the flipper is too close to the playfield it can cause friction between the flipper bat and the flipper bushing. If this heats up during play it can cause a flipper to stick. I would look at those two things first before ordering a rebuild kit.

From Clay's Repair guide

Flipper Gets Stuck in the Up Position...
Before proceeding with repair, first determine if the stuck flipper is a mechanical or
electrical problem. If the flipper is stuck in the "up" position, turn the game off. Does
the flipper return to its normal "down" position? If so, the problem is electrical.
Otherwise if the flipper stays up even when the game is turned off, then the problem
is mechanical.
Electrical: On Deger (Playboy, Monday Night Football) and solid state flipper
games (Robocop and later), check/replace the flipper coil diode. If the flipper
coil diode is broken or shorted on, the flipper coil will stay energized after first
using the flipper(s). Replace this diode with a new 1N4004 or 1N4007 diode.
The counter EMF (Electro-Motive Force) produced by the magnetic field in the
coil collapsing is a reverse polarity voltage. This reverse polarity voltage can
also cause damage to the solidstate flipper board.
If the solidstate flipper(s) are still stuck in the Up position, then check the
solidstate flipper board's TIP36 and TIP32 transistors. The TIP36 transistors
supply the initial power voltage to the flipper coils, and can be easily
damaged. The TIP32 transistors supply the "hold" voltage to the flipper coils
(and hence are not as easily damaged as the TIP36 transistors). Both TIP
transistor types are easily checked with a DMM, see the transistor section for
more details). If a TIP36 has failed, it will usually hold the flipper "up" as soon
as a game is started (or sooner!) If the flippers are still stuck in the up
position, but only after the flipper button is initially pressed, check the S2800B
Silicon Controlled Rectifiers. Often the SCRs will "gate" in the holding voltage
circuit, causing the flipper to remain energized as if the player was holding the
cabinet flipper button in. This was mentioned in service bulletin number 58.

Had this same problem on a TFTC....broken diode leg on the flipper coil. Check the diode on the flipper coil.

How did you measure the diode? Still connected to the coil? If so that tells me the 1st one you checked which measures .6V is not connected...otherwise you'd be reading across the coil windings and reading low like you saw on the second one. You need to lift one leg of a diode to check properly. And since the 1st one checked properly, sounds like one leg is not connected which is causing your flipper to stick. Check carefully...Or using your meter, check continuity of each diode leg to the small copper winding wire going into the coil for each side.

No problem...