Hi @cardan, thanks for bringing this to our attention.
Could you try clearing your browser's cache and/or another browser and then try to re-send the invites?
Are they currently a member of another Family plan, or a Dropbox Business team?
Also, keep in mind that you can copy and send a link invite any time, but you can’t re-send an email invite, you'd need to create a new invite or send a link invite instead.
Keep me posted!