I don't know what source and how much solution strength of nitrogen you are using as foliar spray. My experience is that foliar spray of two to three percent urea solution does not have any adverse effect on leaves. However, the stage of crop does matter as well the hot time of the day. Also avoid foliar sprays when crop needs irrigation.

Besides other factors, the biuret present in the prilled urea is also toxic for leaves. Biuret concentration and thus its injurious effects will increase with increasing urea concentration in the spray solution.

Yes , leaf burn as acommon symptoms ,  are usually obseved in many plants of commercial importance when urea having biuret concentration  is sprayed as foliar spray , even at 1-2% concentration., irrespective of soil type. Such biuret toxicity in actual terms are more common when plants are in pre-evaluation stage. 

leave burn with urea depending on type of plant, growth stage, time of application, ph solution, etc

Mainly Urea or other source of N fertilizer like NaNO3, (NH4)2SO4, UAN has been used for foliar N-application due to its quick absorption resulting in high recoveries of the applied N-fertilizer. Urea being small & neutral molecules with high solubility, it rapidly penetrates the leaf cuticle by lipophilic and stomatal pathways. Inside the leaf, urea hydrolysis is catalyzed by urease, an enzyme that is present in the plant tissues. A very small part of the foliar applied urea hydrolyzed to ammonia & CO2. The scorching appeared as chlorosis of the leaf tip, where NH4+ accumulation is driven by the transpiration stream.

In short, we can say that leaf scorching is promoted by NH4+ accumulation due to a limitation in N assimilation capacity (lack of balance in N uptake & assimilation).