An arrest isn’t magically invalidated if the police don’t read you your rights on the spot.
There could be a whole thread of just misconceptions people have from watching Cop TV.
Miranda rights protect you from self incrimination when being questioned by the police for a crime.
It doesn’t mean they have to Miranda you the second you get arrested. If you aren’t questioned then it doesn’t apply.
People also think it’s illegal not to let them make a phone call after being arrested.