Bug with "last of month" cron expression

I’ve been trying to set a cron expression to schedule on the last sunday of the month. But it does not seem to work on Pipedream.

I tried the following cron expression:
45 18 * * 7L

It looks like it’s going to work, but it doesn’t. Look at “Next event”, it can’t read it. But the text over it seems to be right. Is this a bug?