Agreed. At the least, it definitely appears to be an abugida (or possibly abjad), and the letter shapes are similar to many Brahmic scripts. My first thought was that it looked kind of like Tibetan cursive but wasn't quite the same, and after some searching it does seem like some older Burmese texts also look similar, and Javanese as well.
But, some of what I'm assuming are the vowel marks are throwing me off, since they don't look quite right for any of the writing systems mentioned so far, at least based on the descriptions I can find. Someone mentioned that it looks like the paper might have been photographed from the wrong side, so maybe this is part of the issue.
Yes, this was my feeling as well. It is very rare that calligraphy in a left-to-right writing system will have letters slanted left. It may have been a photograph from the other side. Rice paper being very thin, it might not be obvious which is the right side.
But, some of what I'm assuming are the vowel marks are throwing me off, since they don't look quite right for any of the writing systems mentioned so far, at least based on the descriptions I can find. Someone mentioned that it looks like the paper might have been photographed from the wrong side, so maybe this is part of the issue.