If you have a some kind of foil or other backing thats between the foam and the F26, you might be okay. If you put the adhesive directly on the foam, the foam board may come loose and fall down. I don’t what the timeframe is for that to happen.

I worked for Lowes years ago. Lowes employees must pass training and testing for their department to be assigned to that department. The employees in paint, plumbing, and lumber would be required to know the adhesives and coatings that can be used on foam board and similar products like tub surrounds. The problem comes when employees from other departments help you. Those other employee may not be required to know about adhesives and coatings. I wouldn’t be surprised if an employee from millworks or hardware was helping you and said you could use construction adhesive on foam board. They are technically correct, but employee might not know that there are different construction adhesives for different applications. I worked in Hardware, Tools, and Millworks. I was never required to know about the various adhesives. The way I learned about PL300 was when Lowes sent me to Pella to learn about sealing around Pella windows. Pella showed us multiple methods including a combination of foam board and PL 300.

This video explains the most common adhesives and sealants used in home construction: https://youtu.be/wZhGA-zrN_U

Always follow the instructions and heed the warnings on the container, the manufacturer’s website, and the safety documentation for the product. If you read the tube, the safety documentation, or the Leech Products website for F26, you’ll realize you have 2 problems.

Although F26 is VOC compliant, that doesn’t mean F26 isn’t harmful or dangerous. The fumes released while F26 cures is flammable. The time to cure depends on temperature. The amount of VOCs present during cure time depends on ventilation. The safety documentation states that both ventilation and removing ignition sources is required.

Many construction adhesives react with foam board and tub surrounds. Leech Products safety information has a bold warning that F26 will attack polystyrene foam and tub enclosures. Check the last line in Working Surfaces. I bet that warning is on the tube too.

Use an adhesive for foam board like Leech Products F13 or Loctite PL 300. Using the wrong construction adhesive for foam board will often result not only in the foam board separating from structure but the foam board also ending up in smaller pieces requiring you to recut and reapply more foam board.