I understand that in mathematical writing, it is standard to use 'we' regardless of how many authors there are because it includes the reader(s). In the acknowledgements, however, it seems to me that 'I' should also be acceptable for a single author but someone whose advice I generally trust said that I should use 'we' even for a single author. I am looking for a second opinion because I disagree with this.

My two objections:

1. I have certainly seen papers where the authors write "we thank" or "we would like to thank" in the acknowledgements.
2. While I could write "the author thanks" instead of "I thank," I do not believe that the acknowledgements section needs to be so formal necessarily.

I suppose there could be journals whose editorial styles prefer "the author" to "I" but maybe that does not have to dictate what style I use when posting to the arXiv or in my thesis.