Usually there is difference to due to state and Fannie Mae guidance. A conventional loan that is originated following Fannie Mae guidelines would have 15 day grace period with late fee of 5% of the principal interest (
https://www.fanniemae.com/content/guide/selling/b8/3/02.html). All other loans usually follow state law.
At my former employer we had mortgages with differing grace periods that were inherited through mergers. A decision was made to give all mortgages a 15 grace period. A letter was sent to advise the borrowers.