Something seems off between Sydney Sweeney and Jonathan Davino ... her fiancé was just spotted in Los Angeles, after she'd been hanging with her girlfriends in town for weeks, then left the country without him -- fueling speculation the couple is on the outs.

Jonathan was spotted Wednesday talking to construction contractors at Sydney's $6 million Los Angeles mansion and taking a phone call.

Sydney went to France this week for Paris Fashion Week without Jonathan ... and when she was in L.A. before her trip, she spent several weeks staying at the Beverly Hills Hotel without him too.

TMZ broke the story ... Sydney stayed with some of her friends at the hotel and they gallivanted around the property for almost a month without him ever being spotted, sparking speculation about a potential breakup.

Keep in mind, Sydney and Jonathan were supposed to get married in May but, as we first told you last month, the wedding is postponed.

Sydney's home being a construction zone is a pretty good reason for her to go stay at a hotel ... but Jonathan not staying with her despite them being in the same city raises some red flags.

Plus, property records show the mansion was purchased by a trust Sydney has used in the past for her other estates ... so this isn't a house she bought with Jonathan.

It's been a while since we've seen Sydney and Jonathan together at all ... the last time was in January, so it's been nearly two months at this point.