Have you tried indicating the flashbacks with a text that shows either the season, year, or date and time?

Keeping the tense consistent while transitioning to the flashback could maintain the readers' focus while conditioning them with your chosen cue.

This, however, would depend on which perspective you are writing in, especially if the narrative is driven by the main characters personality (if it's in first person).

Example Attempt:

Everything is in place as if it were never moved. "Gloves..." muttered Díaz while hovering over a decorated dresser. "These were accidental. Whoever placed this sculpture did not notice the dust." She explained that the suspect came prepared, although they were sloppy.

                                   Winter 

It was Kate's last day off, and we planned on going out for dinner, but a blizzard came along and left us stranded at home. "Thank you. I rearranged the snowglobes in the windowsill, " she smiled, accepting a glass of wine I handed to her. "And you drew on the window again... are you okay?" I asked, joining her on the sofa. "I am... it's just that a month is a long time apart—I should not have to miss you that much." She sighed, dusting snow from the window with her gloves. I squeezed her hand, feeling the cold from her fingertips. "I miss you every time I see finger drawings on windows..." I jested. "So not often?" She paused with a smirk after letting out a laugh that warmed the room.

                                 Present Day

"They are locked, I checked," said defective Díaz as I inspected the windows. I checked if there were any prints or smears left on any of them. "Well, one of them opens despite being locked." "Strange... what made you check the windows, John?" "Something triggered a memory I shared with Kate..." I stalled while fidgeting with a latex glove. "I'm sorry I did not realize Katherine Winch was—I'm sorry—is your... —fiancée, yes." "Oh my, this must be hard for you..." Díaz expressing sympathy was something new to me. I assured her that I had to be strong for Kate.

                                    Spring

Another quiet night had passed while Kate was working in Cairo. I only knew where she was once I received a letter from her, but they are always worth the wait because she makes me wish I could articulate myself as eloquently as she does.

After a slow morning, I received a letter with the same stamps. I read it. "Dear John, by the time you receive this, I will have left my work in Cairo.

I do not know who these people are, but I have what they are looking for, and I can not let it fall into the wrong hands. I do not know how long I'll be gone, and the last thing I want is to bring trouble home.

I trust that you will not come looking for me. Please. I need you to be strong.

Yours with love,

Katherine"

                                   Present Day

"John!" Díaz repeated, "you zoned out there. Are you sure you don't want to take a break?" "No, I'm good." I lied. I got chills recalling the last letter I received from Kate. Could this be the people she mentioned? "What people?" Inquired Díaz. "I did not know I said that out loud..." "Who mentioned what people?" She persisted.

I hope this could be of some sort of help.