our rectifiers have zener diodes in them that snub down any voltage over 13.5-14v.
they don't do anything to cause less voltage to be produced like when alternators are used.
you can get rectifiers with mosfet tech, but they more or less work the same with less heat.

Sailor

I took the decompression thing off mine 20,000 miles ago.

replace the relay just for "Keeping it simple" sake !

I thought there is a spring on the starter gear to disengage the starter when the motor starts and exceds the starter speed.   Starter clutch .

If the starter fails to disengage then guess what ?   The starter clutch is "gummed up".   Maybe some "Marvels Mystery Oil" in the crankcase might masterly clean it up ?

https://www.partsfish.com/oemparts/a/suz/50d331f7f8700232d0b3dcd3/starter-clutch

Problem solved!!
Replaced the rectifier/regulator (RR) with a $28 one from Ebay, which keeps the peak DC battery (charging) voltage at 14.1V and no RR leakage. The battery should be happy with that - and there is no more uncontrolled engaging of the starter motor with the bike not turning off, which i attribute to the overvoltage of 18V+, that i had before. Interestingly, a new Suzuki $220 OEM RR provided very little charging with 13.3V peak only.
Still consider the starter relay control circuit not as fail save (very poor and flat out dangerous design by Suzuki) as it may trip when the regulator is failing.
Hope this is useful for someone with the same problem.

Signing off now (over and) out.

Just to set this straight - the starter motor should not be confused with the stator (the battery charging AC generator component)!
The stator is always connected (through rectifier and regulator to the battery). It's not fused either, so if one is dump enough with the wrong polarity, he may blow the rectifier. Post 96 bikes also have the regulator directly connected to the battery, which can be a problem when using a battery charger. The pre 96 bikes have the regulator only turned on with the ignition switch (orange wire). So again, my bike went from no start to no off or the starter turned on randomly when riding, which means it had to be energized. I think the culprit is the starter timing control/decompression circuit, which likely tripped at over-voltages (18+V) out of the regulator fed by the generator, stator, alternator (whatever you want to call it).

Hard to do a post-mortem because the electronics in the RR is embedded in some kind of epoxy.
If one wants to speculate regarding the bad RR or more accurately the bad regulator : a) resulted from a battery charger that had been set at a higher voltage to fix the old battery, or b) the RR could have been bad from the start, or c) it was designed poorly and was failing over time (getting too hot). The old RR was a after market part, although i'm not sure the OEM will last longer.
Would recommend a few tests when installing a new RR, aftermarket or OEM: check for current leakage, i.e. make sure the battery is not discharged by the RR, check the max charging voltage to be between 13.8-14.5 V, check the rectifier for forward continuity.  After that, re-check from time to time. IF the regulator fails w/o noticing your battery will die prematurely.

A second update: It happened again - starter motor runaway or bike won't turn off..... and this time i debugged the problem. After pulling the wires energizing the starter relay solenoid it was still closed turning the starter motor. After removing the cable that runs from the battery to the starter relay it stopped and the starter relay was open (probably it was enough from turning the terminal nut to dislodge the starter relay plunger). So the starter relay contacts either slightly weld together or the terminal nuts were tightened too much such when starting, the plunger got stuck close.
BTW, this is a well know problem with MCs even HDs have the issue. Ordered a new OEM relay. Will see how set fairs.