At the end of the day your expander button is not going deep enough into the case.
Lee dies are made (so are dillon) so that the powder see thru die is also the expander die. They are 1 in the same.
Take a case and no not over flare it but make sure you don't give the case "Just enough flare to have the bullet fit either". You do not want the mouth of the case to look like a funnel from too much flare. Take the expander out of the die and put it back in the expanded case like you did in your pictures above. Take a magic marker and draw a line on the expander where the end of the expanded case is. Pull the expander out of the case and take a gggooooooooooooddddddddddd hard look at how much of that expander is going into you case. Make sure you look at the full diameter body of that expander only, the rounded bottom does nothing for expansion.. Set the expander next to a case and line up the top of the case with the magic marker line. Now mark on the case where the bottom of the expander ends (full diameter only) and put a mark on the case. Take a bullet and set it next to that case and set it so that it has the same oal of your reloads. Now look at where the bottom of that bullet is compared to the expander depth line.
This is telling you how far that bullet has to size the case when seated. Typical lee expanders only size the case to a depth of +/- 1/4".
Cases:
When fired a case will expand to seal the cylinder of your revolver. Then they shrink +/- 1/1000th of an inch to release from the cylinder walls. You should measure a couple fired cases to get an idea of the diameter of your fired case. Take and re-size those same cases and re-measure the diameter. This will tell you how far down your sizing die is sizing your fired cases down. You should also take and measure the diameter of your cases near the bullets base of the loaded rounds. This will tell you how much the bullet has to expand the case when seating.
More often then not reloading dies are walking a fine line with extremely small changes or dimensions having huge impacts on the end results. 1 of the most common things reloads don't think about is using new brass compared to once fired. New brass can take excessive force to reload if it's extremely clean or the inside edge has burrs. When I buy brass from starline I toss it in a media tumble for +/- 10 minutes. This puts a fine dust on the NIB squeaky clean brass and makes them so much easier to reload. The other thing I do is chamfer the inside mouth of all the cases.
Once fired brass is starting to work harden with the 1st firing. Then it gets work hardened a 2nd time when sized. The thicker the brass the bigger the pressure difference is when seating bullets between NIB VS once fired brass.
It seldom is 1 specific thing that causes issues with reloading. Typically there's several underlying minor things when put together add up to somethings wrong.
IMHO:
If you were using a thin brass like remington you wouldn't be having this issue. If you used a better expander plug you wouldn't be having this issue. What you're seeing is common amongst people who swage their own bullets and bullet casters. A link to a custom mold maker website. They also make other products for reloading one of them being expanders. They sell expander buttons that fit the lee universal expander die.
https://noebulletmolds.com/site/product-category/expanders/expander-plug-pistol/
If you look at them you'll see that all of them have 2 things in common. They are 4/1000th's under the bullet's diameter that you are using. And they have long full diameter bodies.
Myself I prefer 3/1000th's under the bullets diameter for cast bullets and 2/1000th's for soft pure lead swaged bullets.
