Inside Sylvester Stallone and Jennifer Flavin's Issues: 'He Tends to Do Things on a Whim' (Source)

When Jennifer Flavin filed for divorce from Sylvester Stallone in a Palm Beach, Florida, courthouse Aug. 19, the move surprised many.

Just three months before, the pair celebrated their 25th wedding anniversary with gushing his-and-hers Instagram posts. And Stallone himself confirmed in July the family was filming an upcoming reality series for Paramount+.

But behind their smiles and happy statements, trouble lurked. Stallone, 76, and Flavin, 54, "are very different people, age-wise and in temperament," according to a Hollywood source who has spent time with the couple. "What was once exciting and challenging is now routine and irritating."

His apparent impetuousness could be a problem, a Flavin source tells PEOPLE. "He tends to do things on a whim without asking her before," says the source. Indeed, Stallone admitted to TMZ he and Flavin had butted heads over his new Rottweiler, Dwight, but denied the dog was the root cause of their split.

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RELATED: Jennifer Flavin's Daughter Called Her 'Strongest Woman' Days Before Divorce from Sylvester Stallone

Nevertheless, adds the Hollywood source, "disagreements on little things add up after so many years together." That echoes what another source previously told PEOPLE: "It really wasn't just one issue that made her file for divorce. They have had lots of issues for years and she just had enough."

In her divorce filing, Flavin claimed "intentional dissipation" of marital assets, as PEOPLE previously reported. Under Florida law, that can include such things as excessive spending, gambling, gift-giving or unnecessary borrowing by a spouse around the time of a split. Flavin is asking that Stallone be prohibited from selling or spending their assets during divorce proceedings, and is also requesting sole use of their $35 million Palm Beach home.

For more on Stallone and Flavin's breakup, pick up this week's issue of PEOPLE, on newsstands Friday.

"Typically, we prepare a marital balance sheet and divide what is on there," high-profile divorce attorney Randy Kessler, who is not working with Stallone or Flavin, tells PEOPLE. "If there has been such dissipation, we often add the dissipated assets as a line item. [The party accused of dissipation] may end up with much less of the assets which remain if the court thinks they gave away their share by dissipation."

But Stallone is fighting Flavin's claim. His legal team responded to her divorce filing Aug. 29, stating in court documents that the actor "has not engaged in any conduct which constitutes intentional dissipation, depletion and/or waste of marital assets." He is also contesting her sole use of the house.