Hello and thanks for the question. Remember that the best person to answer your post surgical questions is the doctor who performed the surgery and your own general practitioner. They know your particulars and are best to guide you. In the meantime, as you know the quadricep tendon is important for stabilization. It attaches to the four muscles around the knee and these are what allow you to extend your leg. Without the quadricep tendon, one can not do this extension.
Did you have a partial or complete tear of the tendon? Recovery from quadricep tendon repair can take 3 to 6 months and for some as long as 9 months. You are still in the recovery phase. I assume you wore your brace post surgery? Do you do physical therapy or have you been doing it post surgery? It's really important to continue that until they release you from the need and at that point, they should give you exercises and stretches to do on your own. Proper rehabilitation of the knee really does require this.
So, you are likely recovering still and would guess that this 'giving way' feeling is the result of that. Please confirm with your doctor though. If the sensation of buckling continues, you may have retorn the tendon, unfortunately. But that is down the road to consider as at this point, this is too soon for it to be ruled out that it is post op recovery issues. There are also other things that cause knee buckling. https://www.healthline.com/health/knee-buckling#plica-syndrome. There are many knee braces on the market. Your orthopedic doctor can probably direct you quite easily and places like Amazon sell them as well. Let us know how you are doing.