I've had this problem on nearly every Japanese bike I've ever owned. I see 3 reasons. One, the screws were overtightened on assembly. Two, they are made of VERY soft metal. Three, while they may look like Phillips screws, they are actually JIS, or Japanese Industrial Standard screws. There is a difference. I now have a set of JIS screwdrivers, but found that a fluted #2 drywall bit works well. It should work on the RE too. Put the bit in the screw and tap on it with a hammer for about 2 minutes before trying to turn it. You have to be very careful with these things. I destroyed a $150 master cylinder once, because I broke off one of the screws even with the reservoir, then cracked the reservoir trying to get it out.
Also, a bit unrelated, but if your bike spends a lot of time in the sun, I would put a piece of aluminum foil tape over the sight glass. It's not real glass but some kind of plastic, and the sun will deteriorate it fairly quickly. I've repaired several master cylinders by filling the hole where the plastic sight glass deteriorated and fell out with JB Weld, and grinding it flat.