The reason seems to be that the sentence consists of three clauses and that infinitive clauses are treated much like subordinate clauses. [1,2]
These are the three clauses:
- The main clause (Hauptsatz), "Bitte gib uns Bescheid".
- The subordinate clause (Nebensatz), "wenn du planst", which is dependent on the main clause.
- The infinitive clause (Infinitivkonstruktion), "teilzunehmen", which is dependent of the subordinate clause.
The infinitive clause is a clause that lacks a subject and whose verb is written "zu" + infinitive at the end. It may contain other types of sentence parts such as objects and adverbs, e.g.:
"Bitte gib uns Bescheid, wenn du planst an der Veranstaltung teilzunehmen."
Whenever the infinitive clause contains more than just "zu" + verb, it is allowed to place a comma:
"Bitte gib uns Bescheid, wenn du planst, an der Veranstaltung teilzunehmen."
This makes the sentence easier to read and makes it more apparent that there are three clauses in the sentence.