I see this problem in few boats , mine included, you need to be sure that the top bolt in the chainplate is in good condition and tight, i mean if is corroded or loose the chainplate work
a bit loose breaking the caulking at the slot, the best way to know is to slack the shroud
and with a plier or something to grab the top of the chainplate , push sideways to see if you feel any excesive shifting or movement, if you dont want to cut the toerail piece , just try this , clean very well inside of the slot , rest of caulk etc.. put some tape around the perimeter of the chainplate inside of the boat
, mix a small batch of epoxy
with a small amount of silica, pour the epoxy
inside of the slot, in any case the epoxy is not going to stick to the chainplate , just enough to fill a bit the gap, leave at the top enough espace for butyl tape, if the top bolt is tight and with the epoxy dry you made the top end of the chainplate a bit stiff, the butyl tape take care of the rest... Cheers.