Microsoft To Do Tasks Disappeared

I had this same problem on my new phone today and found this thread when I was searching for solutions. Was disappointed to read posts spanning several years with no fixes. I resigned myself to re-creating my missing tasks. Thankfully, my computer version of the lists were full and complete - it was only my new phone that had lost some tasks from several lists. I spent time emailing my full lists from my computer to myself thinking I could copy and paste the missing tasks one by one. After adding the first missing task back on to a list on my phone, I suddenly realised that list was now full and complete - and my added task was now in twice! I went to each of the lists that were missing tasks and just added a dummy new task (random letters) and found that once I added a new item, the list updated itself with all of the missing tasks too! Not sure why it wouldn't sync without me adding another task. Anyway, I thought I should share this unexpected fix in case it helps anyone else.